Rotator cuff tears are a common shoulder injury, affecting 30-40% of individuals over age 60 and significantly impacting function and quality of life. Treatment strategies vary widely in invasiveness and cost, and there is significant controversy regarding the optimal treatment strategy. Consequently, shoulder function after rotator cuff surgery varies tremendously, with at least 30% of patients experiencing long-term shoulder disability and worker's compensation claims exceeding $2 billion per year in the U.S. alone. It is believed that the rotator cuff contributes to shoulder strength and provides dynamic glenohumeral joint stability, but accurate measures of in-vivo glenohumeral joint stability do not exist. This study will use a unique, accurate biplane x-ray system to non-invasively measure dynamic glenohumeral joint stability in the repaired and contralateral shoulders of patients having rotator cuff repair surgery. These measurements, along with measures of shoulder strength, will be recorded at 3, 12, and 24 months post-surgery. In addition, dynamic glenohumeral joint stability and shoulder strength will be measured in a control population with no history of shoulder injury or shoulder surgery. The long-term goal of this research program is to develop treatment techniques that restore and maintain shoulder function for patients with rotator cuff tears. The following specific aims will be investigated: 1) determine if rotator cuff surgery restores and maintains dynamic joint stability, 2) determine the relationship between shoulder strength and dynamic joint stability, and 3) determine if dynamic joint stability is predictive of clinical outcome. The central hypothesis is that dynamic joint stability is not completely restored by rotator cuff surgery, thus compromising shoulder function and potentially leading to long-term shoulder disability. This study will provide data that are fundamental to our understanding of rotator cuff function and the effect of rotator cuff surgery on shoulder function. This study and subsequent studies will provide data necessary to form a basis for evaluating surgical procedures and rehabilitation protocols for patients with rotator cuff tears. As the population ages and stays active in later years, normal shoulder function will be critical to maintaining a healthy, active lifestyle. Improving the efficacy of rotator cuff surgery and rehabilitation will also reduce both direct healthcare costs and secondary costs resulting from diminished productivity. vs